1. Technical Field
The present disclosure generally relates to the performance of assays using assay strips, for example chromatographic lateral flow strips.
2. Description of the Related Art
Performance of biological assays has been by greatly facilitated by the introduction of substrates for performing chromatographic assays. Since its conception, numerous substrates have been proposed for performing chromatographic assays. Such substrates are commonly referred to assay test strips or immunochromatographic strips. One type of assay strip is commonly referred to as chromatographic lateral flow strips or lateral flow strips. Other types of assay strips include western blots, southern blots, electrophoresis gels, dot blots, etc. Assay strips may be used for both qualitative and semi-quantitative assays, which typically employ visual detection schemes.
Assay strips typically provide a matrix of material through which a fluid test sample, which may or may not contain an analyte that is being tested for, can flow. In use, a liquid test sample suspected of containing an analyte to be detected is applied to an application zone of the assay strip. In the case of lateral flow strips, the test fluid and analyte suspended or dissolved therein can flow from the application zone to a detection zone, for example via capillary action. The test fluid typically flows horizontally though the matrix (i.e., laterally), although vertical layers may be employed. At the detection zone the appearance or absence of a visible signal (e.g., test results signal line) reveals the presence or absence of the analyte. Assay strips typically visually display two parallel lines, known as capture or signal lines. One of the lines indicates that test strip performance has not been compromised. The second line becomes visible only when the sample contains an amount of analyte in excess of a minimum or threshold concentration. In such assay strips, the capture or signal lines consist of immobilized capture reagents or receptors which are pre-applied to the matrix during manufacture.
In particular, lateral flow type assay strips may include a binding partner that immunospecifically binds the analyte to be detected and which bears a detectable label. The binding may be competitive binding or non-competitive. Competitive assays are particularly suited to detect smaller molecules, such as drugs and drug metabolites. Non-competitive immunoassays are primarily used for detection of large molecules such as proteins, large hormones, or molecules which have multiple binding sites. The detection zone may include a substrate for a label capable of providing a colored response in the presence of the label. The assay strip may also contain a zone in which analyte is immobilized so that labeled binding partner which is not combined due to an absence of analyte in the sample will be captured and prevented from reaching the detection zone. Multi-zone assay strips are also known, which may include a detection zone that contains an immobilized form of a binding substance for a labeled reagent. The labeled reagent bears a detectable chemical group having a detectable physical property so that it does not require a chemical reaction with another substance. Examples of such include colored species, fluorescers, phosphorescent molecules, radioisotopes and electro-active moieties.
Results have traditionally been interpreted visually by the operator. Resulting test and control signal lines vary greatly in intensity, resulting in highly subjective user analysis. Even positive results may be indicated by an extremely faint, but present, test results signal line. In such circumstances, some operators may visually conclude that no test line is present, while other operators may correctly identify the presence of a test line. The issue is further clouded by the nature of assay strips which sometimes contain a high level of background color, that may be incorrectly identified as a positive test line. Thus, assay strips typically provide results which are at best semi-quantitative and are typically subject to variance by the person performing the assay. Since quantification cannot be performed accurately with the naked eye and hence an exact amount of an analyte cannot be determined, application is restricted. Thus, while assay strip formats provide rapid results, are simple to operate, and are more cost-effective than conventional formats, such formats are typically not subject to quantification.
Different approaches to performing assays using assay strips are desirable, particular ones that address some of the above described problems, as well as approaches that address other problems.